Coherent optical communication links at rates of 100 Gbps/λ and higher have been commercially deployed in recent years. These systems heavily rely on power-hungry (e.g., >10 W) digital signal processing (DSP) devices even for cutting-edge CMOS process technologies (e.g., 16 nm linewidths in commercial products). The ability to support unamplified links of up to 80 km at such high rates justifies the cost of powerful DSPs in light of a reduction of other capital expenses and operating costs. On the other hand, the ever-increasing demand for high bandwidth communications within data centers is pushing direct-detection, intensity modulation four-level pulse amplitude modulation (PAM4) schemes to their limits.
For example, IEEE P802.3cd is expected to standardize as one of its PHY options, 100 GBASE-DR, 100 Gb/s serial transmission over one wavelength using PAM4 over of single-mode fiber >500 m. Results from contributors to the IEEE P802.3cd task group, shown in FIG. 1 (IEEE SMF Task Group Contribution by Marco Mazzini (Cisco), August 2014), indicate that 56 Gbaud/112 Gbps PAM4 requires a feed-forward equalizer to open the eye. Although some approaches have demonstrated feasibility, numerous contributions indicate that meeting link budget margins for this type of PHY option remains challenging.
One type of distortion that a polarized optical input beam that passes through an optical fiber plant experiences relates to undesirable changes to the state of polarization (SOP) of the signal that occur during transmission. In order to avoid having to manipulate polarization states in the DSP domain, which would normally be the expectation for a DSP-based coherent receiver, some designers have proposed to implement polarization control by using optical modulators. To facilitate this, a pilot or marker tone is added at the transmitter to label and track one of the phases of the two polarizations (e.g., the x-polarization, in-phase signal branch) as a reference, such that a control loop algorithm running in a low-power CPU can monitor and adjust the polarization states to correct for polarization rotations in two or three degrees of freedom.
The pilot tone (e.g., 50 kHz) that has been superimposed onto the XI tributary at the transmitter is used to recover the state of polarization at the receiver that low-pass filters the XQ, YI, and YQ signals and synchronously detects these signals in the four branches. Thus, the receiver monitors the amplitudes and signs of these signals, while assuming that carrier phase lock has already been achieved. Low speed signal processing can then be used to adjust the polarization angles to reduce the unwanted pilot tone amplitudes, such that the receiver can compensate for polarization rotation in the fiber. However, this approach suffers from drawbacks related to a bootstrap problem, namely that (1) marker tone detection is possible only after carrier phase recovery, and (2) the carrier recovery depends on the polarization states having first been corrected, e.g., to ensure that a QPSK constellation is available for detection.
One proposed solution to alleviate these drawbacks involves a transmit startup protocol, wherein the same data is simultaneously transmitted in each of the two polarization branches. This requirement allows the carrier recovery loop to “see” QPSK modulation regardless of polarization state, such that carrier phase lock can be achieved. In a second step, the polarization recovery loop is enabled, with the expectation that the 50 kHz marker tone will now be found at the expected frequency. However, such a solution is less than ideal because (1) special startup sequences may not be feasible in a system context due to compatibility reasons (e.g., lack of suitable protocols that can facilitate a special startup sequence), and (2) if DQPSK is used, no phase-locked system is present, such that the ability to synchronously detect the pilot tone, which hinges on a phase-locked system, is lost. As a result, each time lock is lost for any reason, the link has to be broken and restarted causing undesirable disruptions to the operation.
Accordingly, what is needed are systems and methods that operate pilot-less and startup sequence-free when deducing the polarization state.